1. Field of the Invention
The present invention relates to liquid crystal display devices and particularly to a liquid crystal display device which displays a large number of gray levels with a high precision.
2. Description of the Background Art
Multiplex drive type liquid crystal display devices are widely utilized as liquid display devices. Such a drive type device uses liquid crystal interposed between two sets of strip electrode groups arranged oppositely to have the directions thereof intersecting with each other. The liquid crystal is driven by a drive circuit connected to each of the strip electrode groups.
Referring to FIGS. 1 to 3, a conventional multiplex drive type liquid crystal display device comprises: glass plates 103 and 104 having polarization plates 101 and 102 on the outer sides thereof, respectively; pixel electrodes 105 and 106 formed on the inner sides of the glass plates 103 and 104, respectively; a liquid crystal 107 enclosed in a space surrounded by the glass plates 103, 104 and a frame 108; a data drive circuit 110 connected to the pixel electrodes 105; and a scanning drive circuit 109 connected to the pixel electrodes 106.
The pixel electrodes 105 connected to the data drive circuit 110 are called data electrodes. The pixel electrodes 106 connected to the scanning drive circuit 109 are called scanning electrodes. Interconnections between the data electrodes 105 and the scanning electrodes 106 constitute pixels 111.
Referring to FIGS. 1 to 3, the operation principle of the conventional device will be described. A twisted nematic type liquid crystal is used in this device. A liquid crystal display device using a twisted nematic type liquid crystal is hereinafter referred to as TN-LCD. The scanning drive circuit 109 selects the scanning electrodes 106 in order and applies successively a pulse voltage thereto. The data drive circuit 110 applies a signal pulse voltage to the data electrodes 105. In each pixel 111 connected to the scanning electrode 106 to which the pulse voltage is applied, the direction of liquid crystal molecules of the liquid crystal changes depending on the potential difference between the scanning electrode 106 and the data electrode 105.
Referring to FIG. 1, light incident on any pixel from the direction A is polarized in advance by polarization plate 101. When the voltage is not applied to the pixel, the polarization plane of the incident light is rotated by the liquid crystal molecules in the liquid crystal 107. Consequently, if the polarizing directions of the polarization plates 102 and 101 are set parallel, the light transmitted by the liquid crystal 107 cannot pass through polarization plate 102. Viewed from the direction opposite to the direction A, this pixel shown in FIG. 1 is in a non-selected state and forms a dark point.
When the voltage is applied to an arbitrary pixel, the liquid crystal molecules in the liquid crystal 107 change their direction in response to a change in electric field. The length direction of those liquid crystal molecules changes to a direction perpendicular to the pixel electrode. Thus, the direction of each liquid crystal molecule changes to be in parallel with the light travelling direction. As a result, the polarization plane of the light travelling in the liquid crystal does not change. The incident light passes through the polarization plate 102. When viewed from the direction opposite to the direction of the arrow A, the pixel 111 is in a selected state and forms a bright point. When the voltage is applied to the pixels selectively, only the selected pixels form bright points and form a contrast between the non-selected pixels. As a result, an image is represented by the combination of the pixels in the selected state and the pixels in the non-selected state.
The above described example was explained by using the TN-LCD. Also in the case of a liquid crystal display device using other types of liquid crystal, the same operation as described above is carried out in principle. As will be understood from the foregoing description, the LCD is in principle suited for binary display.
In a multiplex drive type liquid crystal display device, three methods for representing more than two gray levels for each pixel are proposed. One of the methods utilizes a correspondence relation between the voltage applied to each pixel and the intensity of the transmitted light. The second method is a method of changing the frequency of application of the voltage to a pixel according to the gray level by which the pixel is to be displayed. The third method is to control the ratio between the area in a bright state and the area in a dark state in a pixel.
A relation as shown in FIG. 4 exists between the voltage applied to the liquid crystal and the intensity of the transmitted light. The first method is mainly used in the TN-LCD. This method utilizes the relation as shown in FIG. 8. According to this method, the magnitude of the electric field applied to the liquid crystal of each pixel is controlled in plural manners so that a display with gray levels can be realized.
Referring to FIG. 4, the abscissa represents the voltage applied to a pixel and the ordinate represents the intensity of the transmitted light in the pixel. If the voltage applied to the pixel increases, the transmitted light intensity in the pixel begins to change at a prescribed threshold voltage V.sub.TH. If the applied voltage is higher than the threshold voltage V.sub.TH, the transmitted light intensity increases substantially linearly according to the increase of the applied voltage and it is saturated at a saturation voltage V.sub.S.
In the following, a method of controlling the voltage applied to the pixels for displaying the pixels with eight gray levels will be described. It is assumed that the maximum transmitted light intensity is 1. It is also assumed that the transmitted light intensity corresponding to the threshold voltage V.sub.TH is 0. The difference between V.sub.TH and V.sub.S is divided into seven equal parts, and the start point, the division points and the end point are defined as V.sub.TH =V.sub.0 and V.sub.1, V.sub.2, . . . , V.sub.7 =V.sub.S. In order to display a certain pixel with the fifth gray level, a voltage corresponding to the potential difference V.sub.4 may be applied between the data electrode and scanning electrode of that pixel. The transmitted light intensity in that case will be about 4/7. In the LCD using this method, a display with 16 gray levels can be attained.
The second method for display with gray levels will be described. According to this method, the frequency of application of the voltage to each pixel changes in plural manners, whereby the pixels are displayed with gray levels. This method is mainly used in a super-twisted nematic type liquid crystal display device (hereinafter referred to simply as STN-LCD) and in a ferroelectric liquid crystal display device (hereinafter referred to simply as FLCD).
The number of times one pixel is set in a selected state for a unit period is called the frequency of excitation of the pixel. This frequency is adapted to the gray level with which the corresponding pixel is to be displayed, whereby the pixel is displayed with the desired level. A method for displaying the respective pixels with 256 gray levels will be described in the following.
The brightest gray level is defined as the 256th level and the darkest gray level is defined as the first gray level. The pixel of the 256th gray level is selected 255 times for the unit period, while the pixel of the first gray level is selected 0 times. In other words, the pixel of the first gray level is not selected at all. As for the second to 255th gray levels, the respective pixels are selected (n-1) times for the unit period with respect to the corresponding gray levels n. If the above mentioned unit period is sufficiently short, it appears to the naked eye that the respective pixels are displayed with the gray levels corresponding to the respective frequencies of excitation.
The third method will be described in the following. According to the third method, the areas in the bright state and in the dark state in the respective pixels are controlled. This method is mainly used in FLCD. An example of the device using this method is described for example in "Collection of Papers for the 13th Symposium on Liquid Crystals sponsored by the Japan Society of Applied Physics, the Chemical Society of Japan and the Society of Polymer Science Japan" pages 138 et seq.
FIG. 6 is a schematic sectional view of one pixel used in this method. This pixel includes normal elements described previously at the beginning of the description of the background art. Plural concavo-convex forms having a depth d1 are formed with pitches d2 on a surface of the substrate 103. A pixel electrode 105 is deposited thereon.
Because of the concavo-convex forms, even if the voltage applied to this pixel is uniform in the electrode of one pixel, the intensity of electric field differs dependent on difference in gaps between the respective pixel electrodes. In a display with intermediate gray levels, if a certain voltage is applied, the intensity of the electric field becomes larger than a certain value where the above mentioned gap is smaller than a value determined by the applied voltage, and a bright state is set in that area while the other area remains in a dark state. The transmitted light intensity of the entire area of this pixel appears to be an intermediate level for the naked eye according to the ratio of the areas in the bright state and in the dark state.
As shown in the example of FIG. 7, by changing the applied voltage, it is possible for the transmitted light intensity of the pixel to have a value corresponding to the applied voltage. The display with gray levels by utilizing the above mentioned relation, is based on the operation principle of a liquid crystal display device according to the third method. FIG. 7 is a graph representing the relation between the applied voltage and the transmitted light intensity according to the above mentioned method. It is to be noted that in the LCD of the example shown in this graph, the polarizing directions of the polarization plates 101 and 102 are combined to set a dark state when the voltage is applied and to set a bright state when the application of the voltage is cancelled.
The above-described display with gray levels is mainly utilized for color display. For instance in a computer terminal, it is presently possible to display 16.sup.3, i.e., 4096 different colors, by displaying each of the three primary colors with 16 gray levels. In order to attain a more natural tone, it is necessary to effect display with more than 16 gray levels with high precision. However, any of the above-described three methods for display with gray levels is not suited for high-precision display with a large number of gray levels such as 16 to 256 gray levels.
The first method involves the following disadvantages. According to this method, as shown in FIG. 8, the difference between the transmitted light intensity corresponding to the saturation voltage V.sub.S and the transmitted light intensity corresponding to the threshold voltage V.sub.TH can be divided by a desired number of gray levels to be realized. The difference of the two voltages corresponding to the adjacent gray levels is substantially equal to a value obtained by dividing the difference of the saturation voltage V.sub.S and the threshold voltage V.sub.TH by the desired number of gray levels minus 1. The above mentioned voltage difference becomes smaller according to the increase of the number of gray levels. Accordingly, a large number of gray levels requires a drive technique of extremely high precision.
Even if a drive technique for selecting an applied voltage with high precision is realized, it is necessary for a liquid crystal layer used to have a high evenness. Otherwise, irregularities would occur in the transmitted light intensity in the respective pixels with respect to the same applied voltage, making it difficult to represent correct gray levels. For the above described reasons, it is impossible, with a large number of gray levels such as 16 to 256 levels, to effect a display with high precision according to the first method.
According to the second method, display with desired gray levels should be obtained theoretically. However, in reality, problems as described below are involved. In order to obtain an image without flickering by normal gray level display, the time for scanning all the pixels needs to be less than about 16 msec. This is because 60 frames are displayed for one second in a television set for example. In such a case, each pixel is scanned once at intervals of 16 msec.
According to this second method, it is necessary to further divide each period of 16 msec according to the number of gray levels and scanning needs to be effected by using the divided minimum unit time thus obtained. This period is about 2.3 msec in the case of 8 gray levels. It is about 1.1 msec in the case of 16 gray levels and it is as short as 0.06 msec in the case of 256 gray levels. At present, it is impossible to scan all the pixels for such a short period.
The third method involves the below described problems. According to this third method, gray levels are represented by a ratio of the areas in the bright state and in the dark state in one pixel. However, in the same manner as in the first method, it is necessary to apply, to the corresponding pixel, a voltage regulated with high precision according to a gray level to be represented. For this reason, a drive device having high precision is required. Furthermore, in a display device using this method, it is impossible to control the areas in the bright state and the areas in the dark state in one pixel. Consequently, it is also impossible to effect a precise display with gray levels.